A USB thumb drive with built-in memory devices is very popular nowadays. Almost everyone has at least one of them. As it is well known that the prices per unit memory capacity of thumb drives are dropping dramatically and the memory capacities can not be changed nor expanded when you purchase it. A USB slot, for example, only can accept one thumb drive. If you have several thumb drives, either you have to swap them or you have to buy a multiple-port USB unit. Furthermore, the dimension of the thumb drive is getting smaller and smaller. It can easily get lost.